The previously known machining center of a horizontal type has a horizontally extending machine bed, on which, on the one hand, at least two movable supporting members are connected one upon the other, and on the other hand, a vertical column is fixed. A workpiece to be machined is attached to a pallet surface of a pallet, and this pallet is removably mounted on the uppermost one of the movable supporting members so that the pallet surface is horizontal. A tool for machining the workpiece is attached to a spindle, and a spindle slide supporting this spindle is mounted on the upper portion of the vertical column.
Such a known machining center has several drawbacks. Firstly, since the workpiece is placed on the horizontal surface of the pallet, chips produced during the machining accumulate on the pallet and occasionally on the workpiece itself. Accordingly, the pallet with the workpiece should be cleaned when it is removed from the uppermost movable member. Secondly, since the workpiece is machined in a position above the machine bed, some chips and cutting oil poured to the machining spot between the workpiece and tool during the machining operation fall onto the machine bed, and therefore the machine bed is deformed through the heat involved in the chips and oil. Moreover, since guideways for guiding said movable supporting members and the like are formed in the upper surface of the machine bed, fallen chips and cutting oil entering into said guideways and the like easily cause malfunctioning of, the machine bed and movable supporting members. Further, because of said guideways formed in the upper surface of the machine bed and driving mechanisms arranged on this surface, it is difficult to arrange means for receiving the chips and cutting oil on the machine bed. Thirdly, the workpiece and tool are connected to each other through a structural span extending in a vertical plane and composed of the pallet supporting the workpiece, the several movable supporting members connected one upon the other and supporting the pallet, the machine bed on which the supporting members are connected, the vertical column standing on the machine bed, the spindle slide mounted on the upper portion of the column, and the spindle supported by the spindle slide and supporting the tool, and this structural span is very long according to the known prior art structure. Accordingly, the stiffness of the machine is low and the thermal displacement and the mechanical displacement based upon the cutting resistance between the workpiece and tool are both large.